As of yesterday (July 8th), as reported by JapanToday, eighty-eight people have died and over 50 remain missing, after torrential rains triggered massive flooding and landslides in western Japan, with rescue operations continuing in disaster-hit areas. Evacuation orders or advisories were issued for up to 5.9 million people in 19 prefectures at one point, while over 30,000 people were staying at evacuation centers as of 3 p.m. Sunday, according to the Fire and Disaster Management Agency. This current disaster can already be considered one of the worst flooding and landslide disasters in recent years.

How common is the danger of disasters caused by heavy rain and typhoons in Japan?

Disasters triggered by heavy rain and typhoons occur every year in Japan. Because the country is sandwiched between the continent and the Pacific Ocean, spring and autumn weather patterns often bring heavy rainy, in addition to the rains brought by the typhoon season, which lasts from July to the end of November.

Typhoons approaching or making landfall in Japan can cause heavy rain, floods, wind storms, and storm surges. Much of the country is built on flood plains, and urban areas are especially vulnerable to sudden increases in the water level along rivers and flooding of roads and underpasses. In mountainous regions and residential sub-divisions built on or near hillsides the danger also comes from landslides and debris flows.

“Guerilla” Torrential Rain

You may have heard the term “guerilla rain” or “gerira gouu” or ゲリラ豪雨（ゲリラごうう) used to describe extremely heavy rain in Japan, but this is not a formal meteorological term. “Guerilla rain”, as a term, was coined by the Japanese media to describe sudden torrential rain in a very localized area, whose appearance and movement is difficult to predict.

The Japan Meteorological Agency (JMA) does not use “guerrilla torrential rain” as a term to describe the amount of rainfall. The JMA describes concentrated torrential rain as “local heavy rain” (局地的大雨) or “short burst of strong rain” (短時間強雨).

Overview of Major Disasters Caused by Heavy Rain in Recent Years

Below we give an overview of the most severe disasters caused by heavy rain and typhoons in the past few years.

In Typhoon No. 12, heavy rain, exceeding 1,000-mm in a wide area centered on the Kii Peninsula was recorded.

The Kii Peninsula is the largest peninsula on the island of Honshu. Wakayama prefecture occupies much of the area, including the entire southern part. To the northwest of Wakayama Prefecture is Osaka Prefecture, whose southern part is on the peninsula. East of Osaka Prefecture is Nara Prefecture; farther east is Mie Prefecture. Image: Wikimedia

In Wakayama, Nara, and Mie prefectures, 72 people were killed and 16 people were missing due to sediment-related (landslide) disasters and flooding of rivers.

In addition, flooding of homes and damage to agriculture and the forestry and fisheries industries occurred throughout the entire country, from Shikoku to Hokkaido.

Nationwide, 82 people died, 16 are missing, and 26,102 residential structures were damaged.

July 30 to August 26, 2014: Nationwide Heavy Rain

Due to a continuous onslaught of typhoons and storm fronts, extremely heavy rain fell throughout Japan from July 30th to August 26th, 2014, causing landslides and severe flooding.

On August 24th, Hiroshima prefecture was struck by a series of landslides following heavy rain. Seventy-six people were killed.

Part of the Yagi 3-Chome area in Hiroshima City was destroyed as a result of a landslide. Image: Wikimedia

In Fukuoka, Hyogo, Kyoto, Ishikawa prefectures and Hokkaido another eight people died, and 13,490 residences were damaged.

September 9 to 11, 2015: Kanto and Tohoku Torrential Rain

Due to a number of typhoons and storm fronts, heavy rain fell from western to northern Japan, with especially heavy rainfall in Kanto (which includes Greater Tokyo Area and and the prefectures of Gunma, Tochigi, Ibaraki, Saitama, Tokyo, Chiba, and Kanagawa) and Tohoku (Akita, Aomori, Fukushima, Iwate, Miyagi, and Yamagata prefectures).

Severe flooding along the Kinugawa River caused by heavy rain, September 2015. Source: MLIT

More than 600-mm of rain fell in the Kanto region and more than 500-mm fell in the Tohoku region, with rainfall exceeding twice the average monthly rainfall in September. Heavy rain caused landslides and flooding, leaving eight people dead. Over 19,700 residential structures were also severely damaged.

August 28 to 31, 2016: Heavy rain caused by Typhoon No. 10

Typhoon No. 10 made landfall on August 30th, 2016 in Iwate prefecture, passed through the Tohoku region then moved out to the Sea of Japan. This storm caused heavy rain in a wide swath from the Tohoku region to Hokkaido.

The first floor of a social welfare building in Iwaizumi-cho, Iwate was flooded due to heavy rains. Nine elderly people died. Source: JMA

The resulting landslides and flooding led to the deaths of 22 people and 5 missing in Hokkaido and Tohoku and caused widespread damage. Most of the deaths occurred in Iwaizumi-cho in Iwate prefecture (21 dead or missing), including the deaths of nine elderly people who perished when the Omoto River flooded the first floor of their social welfare facility.

Fifty thousand people were ordered to evacuate. Rainfall of 10.1 centimeters per hour was recorded in the southern region of Kagoshima. In the city of Aso in Kumamoto prefecture, landslides buried several households.

Emergency warning color codes used by the Japan Meteorological Agency (JMA). Three different levels of warning are issued for the three main weather-related threats (tsunami, earthquake, and heavy rain). Source: Nagoya International Center